You can use Master Page under look and  feel section in SharePoint site settings for changing the Master Page for your site.

You can also use power shell script to achieve the same

$web = Get-SPWeb http://laptop16

$web.CustomMasterUrl = "/_catalogs/masterpage/v15.master"

$web.MasterUrl = "/_catalogs/masterpage/V15.master"

$web.Update()

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